Conventionally, semiconductor devices such as semiconductor memories are manufactured by a method using a reduced projection exposure apparatus in which a circuit pattern drawn on a reticle or a mask is projected onto a wafer or the like through a projection optical system to transfer the circuit pattern to the wafer. The smallest size (resolution) of a circuit pattern that can be transferred by the reduced projection exposure apparatus is proportional to the wavelength of light used for exposure. Therefore, the shorter the wavelength, the higher the resolution. Accordingly, the wavelength of light used for exposure is becoming shorter and shorter with the increasing demand for finer semiconductor devices. Thus, progressively shorter wavelengths of ultraviolet light have been put into use for exposure, i.e. KrF excimer laser (wavelength: about 248 nm), and ArF excimer laser (wavelength: about 193 nm). The photolithography using such ultraviolet light, however, cannot comply with the demand for even finer semiconductor devices. Under these circumstances, there has recently been developed a reduced projection exposure apparatus using extreme ultraviolet (EUV) light having a shorter wavelength than those of ultraviolet light, i.e. a wavelength of the order of 10 nm to 15 nm (such a reduced projection exposure apparatus will hereinafter be referred to as “an EUV exposure apparatus”).
A laser-produced plasma (LPP) light source, for example, is used as an EUV light source of an EUV exposure apparatus. The LPP light source utilizes EUV light having a wavelength of the order of 13.5 nm, for example, which is emitted from a high-temperature plasma generated by applying high-intensity pulsed laser to a target material placed in a vacuum chamber. Examples of target materials used for this purpose include xenon (Xe) gas, droplet, cluster, etc. tin (Sn) droplet, and lithium (Li) droplet. The target material is supplied into the vacuum chamber by a droplet generator or other similar means.
In the EUV exposure apparatus, a light generation section having the EUV light source and a section subsequent to the light generation section, in which optical processing, e.g. exposure, is performed by using EUV light generated in the light generation section, are different from each other in service conditions. The LPP light source generates a plasma by applying high-luminance pulsed laser light to a target in an EUV light generation chamber, thereby generating EUV light. During the laser irradiation, scattering particles and ions known as debris are undesirably produced from the target. The debris contaminates and damages a mirror that converges EUV light, causing a degradation of the reflectance. To reduce the degradation of the reflectance of the EUV light converging mirror by the debris, a buffer gas, e.g. He, is conventionally supplied into the light generation chamber. Accordingly, the pressure in the light generation chamber is about 10 Pa.
On the other hand, the pressure in an apparatus that applies EUV light to a mask to perform exposure is required to be about 10−7 Pa. Patent Literature 1 (FIG. 12) proposes a differential evacuation system that realizes the pressure difference between the light generation chamber and the exposure apparatus. An EUV exposure apparatus in Patent Literature 1 includes a light generation chamber having an EUV light source and an illumination optical chamber in which optical processing, e.g. exposure, is performed by using light generated in the light generation chamber. A turbomolecular pump is installed between the light generation chamber and the illumination optical chamber. The rotating shaft of the turbomolecular pump is made hollow to allow light to pass through the hollow inside of the rotating shaft, thereby forming a chamber connecting passage. The two chambers are evacuated individually, and at the same time, the turbomolecular pump is driven to evacuate gas molecules leaking through the chamber connecting passage from the high-pressure side chamber toward the low-pressure side chamber, thereby introducing light generated in the light generation chamber into the illumination optical chamber through the chamber connecting passage while maintaining a large pressure difference between the two chambers.
The reason why the chamber connecting passage is evacuated by the turbomolecular pump installed between the two chambers as stated above is that a large pressure difference cannot be maintained between the two chambers simply by connecting together the two chambers, which are evacuated individually, through the chamber connecting passage. It should be noted that the chamber connecting passage cannot be closed with a filter because EUV light is passed therethrough (it is difficult to produce a filter material having a high transmittance in the wavelength region of EUV light).
In the differential evacuation system disclosed in the following Patent Literature 1, however, only one turbomolecular pump can be installed. Therefore, when a large differential pressure is required, it is necessary to greatly increase the external size of the turbomolecular pump to increase the pump capacity, or to reduce the conductance of the chamber connecting passage provided in the turbomolecular pump (i.e. the passage diameter is reduced to increase the resistance to the passage of gas molecules). However, the turbomolecular pump has a special structure and hence a high production cost. If the turbomolecular pump is increased in size, the production cost becomes higher. On the other hand, it is difficult to reduce the conductance because it is necessary to sufficiently ensure a desired optical path.    [Patent Literature 1] Japanese Patent Application Publication No. 2004-103731